An image forming apparatus which includes a plurality of light sources corresponding to each developing unit, and a multi-beam scanning unit configured to form electrostatic latent images on a photosensitive member with a plurality of light beams respectively emitted from a plurality of light sources has been hitherto known. In this image forming apparatus, the electrostatic latent image forming interval between the light sources which is the interval between the electrostatic latent images formed with a plurality of light beams corresponding to each developing unit may fluctuate due to optical errors, mechanical errors, fluctuations in optical systems with an increase in temperature, or the like, and image quality may be degraded.
Accordingly, an image forming apparatus which has a function of adjusting the electrostatic latent image forming interval between the light sources has been hitherto known (see, for example, JP-A-2004-098593). Specifically, this image forming apparatus causes a multi-beam scanning unit to perform an operation to form so-called solid marks with no gap between scanning lines only with light beams from the same light source for each of a plurality of light sources. The image forming apparatus has a sensor which outputs a signal according to the positions of a plurality of marks formed on a photosensitive member, and adjusts the electrostatic latent image forming interval between the light sources based on the signal from the sensor.
However, in the image forming apparatus of the related art described above, in order to adjust the electrostatic latent image forming interval between the light sources, it is necessary to form solid marks with light beams from the same light source. The fluctuation in the electrostatic latent image forming interval between the light sources may have an influence on processing for adjusting the forming positions of the electrostatic latent images by a plurality of light sources which are the positions of the electrostatic latent images formed with a plurality of light beams from a plurality of light sources. For this reason, there has been demand for improvement.